The following description is used to describe the background and products, materials and production methods that may comprise specific parts of preferred embodiments in the disclosure of this invention.
The majority of all laminate floors are produced according to a production method generally referred to as Direct Pressed Laminate (DPL). Such laminated floors have a core of 6-12 mm fibreboard, a 0.2 mm thick upper decorative surface layer of laminate and a 0.1-0.2 mm thick lower balancing layer of laminate, plastic, paper or like materials.
The surface layer of a laminate floor is characterized in that the decorative and wear properties are generally obtained with two separate layers of paper, one above the other. The decorative layer is generally a printed paper and the wear layer is a transparent overlay paper, which comprises small aluminium oxide particles.
The printed decorative paper and the overlay are impregnated with melamine formaldehyde resins, generally referred to as melamine resins. The impregnation is based on a two-step process where the paper in a first step passes a bath of liquid melamine resin and in a second step is dried to a sheet that is completely impregnated and covered with a dry melamine resin. The decor paper that prior to impregnation has a weight of 60-80 gr/m2 comprises generally about 50 wt % of melamine formaldehyde thermosetting resins. The resin content in the overlay may be even higher. Aluminium oxide particles are applied and incorporated during impregnation into the resin layer on one side of the overlay paper that during pressing is in contact with the decor paper. The impregnated papers are laminated to a HDF core in large discontinuous or continuous laminate presses where the resin cures under high heat (about 170° C.) and pressure (40-60 bars) and the papers are laminated to the core material. An embossed press plate or steal belt forms the surface structure.
Digital printing may be used to print an image on the decorative paper sheet or on the overlay. The digital printing is generally made prior to impregnation and the flexibility that the digital technology provides cannot be fully utilized. It would be an advantage if digital print may be made after impregnation and on an impregnated paper or if impregnation could be avoided. Printing directly on a melamine impregnated paper is difficult since the ink drops applied on the melamine layer bleed during printing and especially when the dry melamine layer becomes liquid and floats during the pressing operation.
Laminate floors may also have a surface of coated paper, foils or plastic foils and such foil materials are difficult to print digitally. A protective wear resistant transparent layer that generally is a polyurethane lacquer is used to covers the printed décor.
Luxury Vinyl Tiles, generally referred to as LVT floorings, are designed as a layered product made of thermoplastic PVC mixed with plasticizers. The name LVT is somewhat misleading since a major part of LVT floors have a plank size with a wood pattern.
Thermal moulding based on calendar rolling or extrusion is used to form the PVC layers. During the calendar rolling, the PVC material is heated to its softening temperature and exposed to pressure between cylinders and cooled down.
The base layer or the core is made primarily of PVC mixed with chalk and/or limestone fillers and has a high quality printed decorative PVC foil on the upper side. A transparent wear layer of vinyl with a thickness of 0.2-0.6 mm is generally applied on the decorative foil. When PVC is heated it becomes soft like a paste and bonds under heat and pressure to other PVC materials but also to organic and inorganic fibres or minerals when cooled down to room temperature. The base layer, the decorative foil and the transparent layer are fused or laminated together with heat and pressure in continuous or discontinuous press operations. The laminated sheets are after pressing annealed to remove stress and to achieve increased dimensional stability. Calendar rolling, pressing and annealing general takes pace at temperatures between 120° C.-160° C. Annealing may be combined with ageing at temperatures around 25° C.-30° C. during a few days.
The decorative effects are obtained with a white decorative foil that covers a dark colour of the core and provides a base colour for the rotogravure printing process where special solvent based inks are used that bond to the PVC surface. Such inks are difficult to combine with an environmental friendly, flexible and cost efficient printing process, especially when a digital printing method is used.
LVT floors offer several advantages over for example laminate floors such as deep embossing, flexibility, dimensional stability related to humidity, moisture resistance and lower sound. Digital printing of LVT floors is only on an experimental stage since it is difficult to print on the plastic foil but would, if introduced, provide major advantages over conventional printing technology.
As a summary it may be mentioned that only small volumes of floor panels are printed digitally, especially vinyl and laminate floorings, mainly due to high cost of ink and high investment cost for the industrial printers but also due to the fact that it is difficult to apply a digital print on the specific surface materials used in such flooring applications.